Delta-sigma modulators (noise shapers) are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, a delta-sigma modulator spreads quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, a delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
In addition to data conversion applications, delta-sigma noise shapers are increasingly utilized in the design of digital amplifiers. In one particular technique, a digital delta-sigma noise shaper provides a noise shaped (quantized) digital data stream to a pulse width (duty cycle) modulator PWM, which in turn drives a linear amplifier output stage and associated load. This technique is generally described in U.S. Pat. No. 5,784,017 entitled “Analogue and Digital Convertors Using Pulse Edge Modulators with Non-linearity Error Correction” granted Jul. 21, 1998 and U.S. Pat. No. 5,548,286 entitled “Analogue and Digital Convertor Using Pulse Edge Modulators with Non-linearity Error Correction” granted Aug. 20, 1996, both to Craven, U.S. Pat. No. 5,815,102 entitled “Delta Sigma PWM DAC to Reduce Switching” granted Sep. 29, 1998 to the present inventor (incorporated herein by reference), U.S. patent application Ser. No. 09/163,235 to the present inventor (incorporated herein by reference), and International Patent Application No. PCT/DK97/00133 by Risbo.
One difficulty in implementing these digital amplifiers is minimizing noise and distortion due to power supply noise and variations. This problem is correctly identified in U.S. Pat. No. 5,559,467 to Smedly (“the '467 patent”). Specifically, the '467 patent recognizes the need to account for the time-varying value of the power supply voltage during modulation; however, the solution proposed in the '467 patent introduces its own distortion into the system.
Hence, improved circuits and methods are required for minimizing noise and distortion in digital amplifiers in light of power supply noise and time variations.